Re: [PATCH] mm/zswap: fix potential deadlock in zswap_frontswap_store()

[Vlastimil Babka]

On 04/03/2017 02:38 PM, Andrey Ryabinin wrote:
> 
> 
> On 04/03/2017 03:37 PM, Andrey Ryabinin wrote:
>>
>>
>> On 04/03/2017 11:47 AM, Michal Hocko wrote:
>>> On Fri 31-03-17 10:00:30, Shakeel Butt wrote:
>>>> On Fri, Mar 31, 2017 at 8:30 AM, Andrey Ryabinin
>>>> <aryabinin@xxxxxxxxxxxxx> wrote:
>>>>> zswap_frontswap_store() is called during memory reclaim from
>>>>> __frontswap_store() from swap_writepage() from shrink_page_list().
>>>>> This may happen in NOFS context, thus zswap shouldn't use __GFP_FS,
>>>>> otherwise we may renter into fs code and deadlock.
>>>>> zswap_frontswap_store() also shouldn't use __GFP_IO to avoid recursion
>>>>> into itself.
>>>>>
>>>>
>>>> Is it possible to enter fs code (or IO) from zswap_frontswap_store()
>>>> other than recursive memory reclaim? However recursive memory reclaim
>>>> is protected through PF_MEMALLOC task flag. The change seems fine but
>>>> IMHO reasoning needs an update. Adding Michal for expert opinion.
>>>
>>> Yes this is true. 
>>
>> Actually, no. I think we have a bug in allocator which may lead to recursive direct reclaim.
>>
>> E.g. for costly order allocations (or order > 0 && ac->migratetype != MIGRATE_MOVABLE)
>> with __GFP_NOMEMALLOC (gfp_pfmemalloc_allowed() returns false)
>> __alloc_pages_slowpath() may call __alloc_pages_direct_compact() and unconditionally clear PF_MEMALLOC:
>>
>> __alloc_pages_direct_compact():
>> ...
>> 	current->flags |= PF_MEMALLOC;
>> 	*compact_result = try_to_compact_pages(gfp_mask, order, alloc_flags, ac,
>> 									prio);
>> 	current->flags &= ~PF_MEMALLOC;
>>
>>
>>
>> And later in __alloc_pages_slowpath():
>>
>> 	/* Avoid recursion of direct reclaim */
>> 	if (current->flags & PF_MEMALLOC)        <=== false
>> 		goto nopage;
>>
>> 	/* Try direct reclaim and then allocating */
>> 	page = __alloc_pages_direct_reclaim(gfp_mask, order, alloc_flags, ac,
>> 							&did_some_progress);
>>
> 
> 
> Seems it was broken by
> 
> a8161d1ed6098506303c65b3701dedba876df42a
> Author: Vlastimil Babka <vbabka@xxxxxxx>
> Date:   Thu Jul 28 15:49:19 2016 -0700
> 
>     mm, page_alloc: restructure direct compaction handling in slowpath

Yeah, looks like previously the code subtly relied on compaction being
called only after the PF_MEMALLOC -> goto nopage check and I didn't
notice it. Tell me if I should add a check or you plan to send a patch.
Thanks!

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